Controlled vent for isotope fuel capsule

ABSTRACT

A controlled vent for an isotope fuel capsule utilizes physical passages for gas flow which are large enough to permit flow to take place be viscous or free molecular flow and are small enough or sufficiently tortuous to prevent passage of fuel particles. A plurality of punctured thin metal discs or foils are stacked together with the holes misaligned to form a continuous flow channel, thereby permitting gas generated within the capsule to escape through the vent. Thus, the weight and volume of the fuel capsule can be reduced since the internal pressure is reduced.

United States Patent lnventor Alfred E. Cottam Bethel Park, Pa.

Appl. No. 825,779

Filed May 19, 1969 Patented Jan. 4, 1972 Assignee Westinghouse ElectricCorporation Pittsburgh, Pa.

CONTROLLED VENT FOR ISOTOPE FUEL CAPSULE 9 Claims, 4 Drawing Figs.

US. Cl 250/ 106 S, 176/67 Int. Cl G211: 5/00 Field of Search 250/106 S;176/66, 67

[56] References Cited UNITED STATES PATENTS 2,830,190 4/1958 Karp250/106 S 3,133,196 5/1964 Rochlin 250/106 S 3,488,502 1/1970 Dukes250/106 S Primary Examiner-Archie R. Borchelt AttomeysA. T. Stratton andZ. L. Dermer ABSTRACT; A controlled vent for an isotope fuel capsuleutilizes physical passages for gas flow which are large enough to permitflow to take place be viscous or free molecular flow and are smallenough or sufficiently tortuous to prevent passage of fuel particles. Aplurality of punctured thin metal discs or foils are stacked togetherwith the holes misaligned to form a continuous flow channel, therebypermitting gas generated within the capsule to escape through the vent.Thus, the weight and volume of the fuel capsule can be reduced since theinternal pressure is reduced.

mmmm 4m l 3l SINGLE ORFICE 3: PER use 3 (I) (1) LL] D: D. 5 A 3 TWOORFICES a PER DISC 24 0 U1 9 FOUR ORFICES 2 PER DISC FIGS.

EQUIVALENT LEAK PASSAGE LENGTH INVENTOR WITNESSES 7 Alfred E. Cottom BYI I I 27mm ATTORNEY CONTROLLED VENT FOR ISOTOPE FUEL CAPSULE BACKGROUNDOF THE INVENTION This invention relates, generally, to radioisotopes andmore particularly, to fuel capsules for radioisotopes utilized as asupplementary source of electrical power for space vehicles.

When a fuel, such as plutonium-238 which generates helium gas, isutilized in a radioisotope capsule, the safety aspects of the capsuleare important. Conventional capsules relay on a heavy structuralrefractory metal shell to contain the high internal pressures generatedby the fuel and to ensure capsule survival and fuel containment afterimpact.

An object of this invention is to provide for venting gas generated byfuel in a radioisotope capsule while containing the fuel particleswithin the capsule.

Another object of the invention is to reduce dependence on the long termrupture characteristics of the structural shell of the fuel capsule.

A further object of the invention is to enhance impact survival bysubstantially reducing the fuel capsule internal pressure.

Still another object of the invention is to prevent oxidation of therefractory metal shell through oxygen diffusion.

Other objects of the invention will be explained hereinafter or will beapparent to those skilled in the art.

SUMMARY OF THE INVENTION In accordance with one embodiment of theinvention, a controlled vent for a radioisotope fuel capsule comprises aplurality of thin punctured metal discs which are stacked one uponanother with the holes through the discs misaligned. After stacking, thediscs are welded together at the periphery to form a solid cylinderwhich is secured in a wall of the cap sule. Thus, physical passages areprovided for gas flow which are large enough to permit flow to takeplace by viscous or free molecular flow and are small enough orsufficiently tortuous to prevent passage of fuel particles. The leakagecan be controlled as a function of hole diameter, number of layers andnumber of holes in each disc.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thenature and objects of the invention, reference may be had to thefollowing detailed description, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a diagrammatic view, partly in section, of a radioisotopegenerator with a fuel capsule having a controlled vent embodyingprincipal features of the invention;

FIG. 2 is an exploded view, showing the manner of stacking the discsutilized in the vent;

FIG. 3 is an enlarged view, in section, of the discs in assembledrelation, and

FIG. 4 is a graphical representation showing the relation between insidecapsule pressure and equivalent passage length for different numbers oforifices in the discs utilized in the vent.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, andparticularly to FIG. 1, the radioisotope generator shown therein may beof a type well known in the art suitable for use in a space vehicle. Asshown, the generator 10 is generally cylindrical in shape with closedends. The generator may comprise an outer metal shell 11, a shield 12, afuel capsule 13 containing a suitable fuel 14, such as plutonium-238,and a plurality of thermoelectric converters 15 disposed between theshield 12 and the capsule l3 and surrounded by thermal insulation 16.The thermoelectric converters l5 transform part of the isotope decayheat into electricity in a manner well known in the art. The remainingheat energy produced by the fuel flows into the outer shell from whichit is radiated or conducted to the surrounding air or space.

As explained hereinbefore, when plutonium-238 is utilized as a fuel,helium gas is generated within the capsule 13. In order to prevent anexcessive pressure within capsule, a controlled vent 21 is provided forthe capsule. The vent 21 provides physical passages for gas flow whichare large enough to permit flow to take place by viscous or freemolecular flow and are small enough or sufficiently tortuous to preventpassage of fuel particles.

As shown more clearly in FIGS. 2 and 3, the vent 21 comprises aplurality of thin punctured metal discs or foils 22 which are stackedone upon another with the holes in the disc misaligned. Each disc 22 hasone or more holes 23 therethrough which are of a small diameter, forexample 0.0005 inches. By way of example, each disc 22 may be 0.005inches thick. The holes may be drilled in the discs by an electron beamor laser, or other suitable means.

As shown in FIG. 2, each disc contains two pairs of diametricallyopposed holes 23 and holes 23 are spaced substantially apart. As shown,the discs are stacked with the holes in alternate discs rotatedsubstantially 45. After being stacked, the discs are welded together atthe periphery, as shown at 24 in FIG. 3, to form a solid cylinder. Theassembled vent 21 is then secured in a wall of the capsule 13, as bywelding.

The discs 22 may be composed of platinum alloys with rhodium, therebyproviding a stable vent, free from oxidation and compatibility problemsafter reentry and impact. The permeation of gas through the controlledvent depends on the flow regime, which, in turn, is determined by thepore size, pressure, and leak rate. If calculated as a labyrinth seal,leakage can be controlled as a function of hole diameter and number oflayers.

Each curve in FIG. 4 shown internal capsule pressure versus leak passagelength in which the effective path length resulting from a geometricalarrangement has an effect similar to a labyrinth seal. The evaluation ofthe seal depends on the clearance between the layers and the complicatedpath involved. An estimation of the pressure drop due to this effect ismade using the equation for a labyrinth seal.

The different curves in FIG. 4 show the results obtained with a singleorifice per disc, two orifices per disc and four orifices per disc. Inthis manner, the leakage of gas from inside the capsule 13 may becontrolled by means of the vent 21. A vent 25, which may be of aconventional type, may be provided in the shield 12 and outer shell 11to permit the gas to escape from within the shield 12.

From the foregoing description, it is apparent that the inventionprovides a controlled vent for reducing the pressure within an isotopefuel capsule, thereby permitting a reduction in the size and weight ofthe capsule without reducing the safety features of the capsule. Thecontrolled vent permits leakage of a gas without permitting passage offuel particles through the vent. Desired leakage rates may be obtainedby varying the number of discs, the number of holes and the size of theholes in the discs utilized in the vent structure.

Since numerous changes may be made in the abovedescribed structure, anddifferent embodiments of the invention may be made without departingfrom the spirit and scope thereof, it is intended that all subjectmatter contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

I claim as my invention:

1. In a radioisotope generator, in combination, a capsule containing aradioisotope fuel, a controlled vent for the capsule, said ventcomprising a plurality of punctured metal discs stacked one uponanother, and means retaining said discs in a wall of the capsule.

2. The combination defined in claim 1, wherein each disc has at leastone hole therethrough and the discs are assembled with the holes inmisalignment.

3. The combination defined in claim 2, including means retaining thediscs in said assembled relationship.

4. The combination defined in claim 2, wherein the discs are secured inassembled relationship by being welded together at their periphery.

5. The combination defined in claim ll, wherein each disc has at leasttwo diametrically opposed holes therethrough and the discs are assembledwith the holes in alternate discs angularly rotated. I

6. The combination defined in claim ll, wherein each disc has two pairsof diametrically opposed holes therethrough spaced substantially 90apart.

7. The combination defined in claim 6, wherein the discs are assembledwith the holes in alternate discs angularly rotated substantially 45.

8. A controlled vent for a capsule containing a radioisotope fuel, saidvent comprising a plurality of puncturcdmetal discs stacked one uponanother with the holes through the discs in misalignment, and meansretaining said discs in a wall of the capsule.

9. The vent defined in claim 8, including means retaining the discs insaid assembled relationship. 1

1. In a radioisotope generator, in combination, a capsule containing aradioisotope fuel, a controlled vent for the capsule, said ventcomprising a plurality of punctured metal discs stacked one uponanother, and means retaining said discs in a wall of the capsule.
 2. Thecombination defined in claim 1, wherein each disc has at least one holetherethrough and the discs are assembled with the holes in misalignment.3. The combination defined in claim 2, including means retaining thediscs in said assembled relationship.
 4. The combination defined inclaim 2, wherein the discs are secured in assembled relationship bybeing welded together at their periphery.
 5. The combination defined inclaim 1, wherein each disc has at least two diametrically opposed holestherethrough and the discs are assembled with the holes in alternatediscs angularly rotated.
 6. The combination defined in claim 1, whereineach disc has two pairs of diametrically opposed holes therethroughspaced substantially 90* apart.
 7. The combination defined in claim 6,wherein the discs are assembled with the holes in alternate discsangularly rotated substantially 45*.
 8. A controlled vent for a capsulecontaining a radioisotope fuel, said vent comprising a plurality ofpunctured metal discs stacked one upon another with the holes throughthe discs in misalignment, and means retaining said discs in a wall ofthe capsule.
 9. The vent defined in claim 8, including means retainingthe discs in said assembled relationship.